Three months is not a bad wait time for a piece to get done. There are a lot of items to factor in. If the piece is not detailed, and is very plain, then three months is a bit much. While I do not have much time on the boards, I have a decent amount of experience.
Things to consider:
1. The artist (abilities, details given to pieces)
2. Artists's obligations. When the big boys call up and want panels for publishing, commissions are normally put on pause or halted.
3. Conventions and other commission requests. If the artists travels for conventions, you may as well count the weeks surrounding the convention a wash. Your piece might be worked on to strike interest for more commissions at the CON, but more likely, it will be left safely at the artist's studio/home.
4. Life. Life happens. These men/women have a livelihood to keep and progress. Often with families as well. Your piece my be paid for in advance, but variables occur, and sometimes that means your piece has to be put to the side.